As of Patch 2.0.3 To unlearn Dragonscale Leatherworking, Peter will tell you to first go see Narain Soothfancy in the Tanaris Desert, just north of the Steamwheedle Port. Narain points you to the book on the table which will teach you either Dragonscale, Elemental or Tribal leatherworking.

You can also unlearn your Dragonscale specialization with Peter, but it costs 100.